Recently, as a vehicle (for example, an electric car) using a new type of fuel has appeared, the weight of a vehicle fuel system such as a storage battery is expected to be increased in comparison to the present internal combustion engine, and therefore, the development of a lightweight material that may significantly reduce the weight of a vehicle body is being required.
As a lightweight material, use of aluminum (Al) or magnesium (Mg) is under discussion, but Al or Mg has a low degree of strength and ductility, and incurs high costs. Thus, steel is still inevitably used.
Steel has more improved strength and ductility than those of Al or Mg, and also has lower costs than those of Al or Mg. Vehicle bodies have heretofore been made lightweight by reducing the thicknesses of a high-strength, high-toughness steel, but when a high specific gravity of the steel itself does not meet the limitation of weight lightening required for vehicles, a nonferrous metal such as Al is inevitably used in the steel.
Accordingly, steel having its specific gravity reduced by primarily adding Al, a light element, is being developed. A technique of manufacturing ferritic steel in which Al of 2.0 to 10.0 wt % is added to ultra-low carbon steel and a technique of manufacturing austenitic steel in which Al of 8 wt % and manganese (Mn) of 10 to 30 wt % are added to ultra-low carbon steel have been known.
The ferritic steel has a problem in that carbon of 0.2 wt % or less and aluminum of 2.5 wt % to 10 wt % are added thereto by a means of technology (Patent Document 1) which includes carbon of 0.8 wt % to 1.2 wt %, manganese of 10 wt % to 30 wt %, and aluminum of 8 wt % to 12 wt %, rigidity and a certain degree of ductility are obtained through the control of a precipitate and a crystal texture, but tensile strength is reduced to about 400 MPa and an elongation percentage is only about 25%.
To solve this problem, a dual phase lightweight steel sheet having no ridging and having improved strength and ductility was developed by containing a large amount of residual austenite to cause transformation induced plasticity and controlling the crystal texture of ferrite (Patent Document 2).
However, when the dual phase lightweight steel sheet is reheated to hot roll a slab, or thermally treated to obtain mechanical properties, the dual phase lightweight steel sheet is decarbonized and causes a problem in that the amount of austenite is reduced along with the loss of carbon, thus decreasing strength and ductility.
Patent Document 1: Japanese Patent Laid-Open No. 2006-176843
Patent Document 2: Japanese Patent Laid-Open No. 2009-287114